41st Lunar and Planetary Science Conference (2010)
2696.pdf
The LASP Lunar Albedo Measurement and Analysis from SOLSTICE
(LLAMAS) Greg Holsclaw1 , Martin Snow1 , Amanda Hendrix2 , and William McClintock1 , 1 Laboratory
for Atmospheric and Space Physics/University of Colorado, 1234 Innovation Dr, Boulder, CO 80303
([email protected]) and 2 Jet Propulsion Laboratory/California Institude of Technology, 4800 Oak
Grove Dr., MS 230-250, Pasadena, CA, 91109
Introduction
The SOLar-STellar Irradiance Comparison Experiment
(SOLSTICE) [1] on the SOLar Radiation and Climate
Experiment (SORCE) [2] has been observing the lunar
irradiance in the ultraviolet on a routine basis since June
2006. SOLSTICE is a two channel grating spectrometer that monitors the solar irradiance from 115 to 300 nm
and uses an ensemble of bright stars to track instrument
degradation. The far ultraviolet (FUV) channel measures
from 115 to 180 nm, while the middle ultraviolet (MUV)
channel measures from 180 to 300 nm. The large dynamic range of the instrument allows us to also measure the lunar irradiance with good signal-to-noise over
a wide range of phase angles.
Photometric measurements of atmosphereless solar
system bodies provide information on the textural and topographic properties of their surfaces. Near zero phase,
the disk-integrated reflectance rises sharply. This phenomenon is commonly known as the opposition surge.
The amplitude of the rise and its width in phase angle
provide information about the porosity, grain size distribution, and the physical mechanism of reflection [3].
Disk-integrated measurements at large phase angles provide information on topographic surface roughness [4].
SOLSTICE observations of the Moon are well-suited to
such photometric studies as they span a wide range of
phase angles (∼ 0◦ – 170◦ ). The ultraviolet wavelength
range of SOLSTICE may provide unique information on
these photometric properties of the Moon.
Unlike previous ultraviolet albedo measurements, the
SOLSTICE instrument observes both the lunar and solar
irradiances. As described in Snow et al. [5], this observing technique removes uncertainties due to instrument cross-calibration in the measurement of the diskintegrated lunar albedo. An additional advantage to using SOLSTICE to observe the Moon is that we have solar
spectra taken shortly before or after the lunar spectrum
(usually less than an hour). This is particularly important in the FUV band where the Sun is highly variable
compared to the visible.
Observations
During the daylight portion of each orbit, SOLSTICE
measures the solar irradiance at 0.1 nm spectral resolution. When the spacecraft is in eclipse, the instrument exchanges the small solar entrance and exit slits for much
larger stellar apertures. With the increased throughput,
Figure 1: Histogram of SOLSTICE lunar observations.
SOLSTICE can measure the irradiance from the Moon
over the entire 115-300 nm range. The spectral resolution of the lunar observations is 1.1 nm in the FUV and
only 2.2 nm in the MUV.
Figure 1 show the distribution of phase angles that
have been observed so far in the program. Lunar observations compete with the stellar calibration observations for spacecraft time, but we have averaged about
one spectral scan of either the FUV or MUV band per
day. We have purposefully weighted the scheduling priority of lunar observations towards small phase angles
to improve sampling of the phase curve near full Moon.
Starting in 2009, we also began increased observing frequency at large phase angles, so in the future these histograms will also show a peak near 170◦ , the limit of
spacecraft pointing restrictions.
Preliminary Results
The SOLSTICE measurements of both the Sun and
Moon are disk integrated irradiances, therefore the natural quantity to compute from these two datasets is the
disk-equivalent albedo as defined by Kieffer and Stone
[6]:
AM (α) =
EM /ΩM
,
E¯ /Ω¯
(1)
where EM and E¯ are the irradiances of the Sun and
Moon respectively, and Ω is the solid angle subtended by
each object. The disk-integrated albedo at 0◦ phase is
also known as the geometric albedo.
41st Lunar and Planetary Science Conference (2010)
2696.pdf
2
Figure 2: Lunar disk-equivalent albedo at phase angle
of 10 degrees. Blue and red points are from the FUV
and MUV channels respectively. Error bars are due to
systematic and statistical sources [5].
Figure 3: Phase Curve of the Moon at a wavelength of
282 nm. Negative angles correspond to waxing phase,
positive to waning phase.
References
Using only the observations near 10 degrees phase
angle, the disk-equivalent albedo as a function of wavelength is shown in Figure 2. This plot shows only wavelengths longer than 130 nm. Since the SORCE spacecraft
is in low Earth orbit, observations in the neighborhood
of Lyman alpha (121.6 nm) are impacted by geocoronal
airglow emission. We are still in the process of modeling
this contribution to estimate the lunar albedo at Lyman
alpha.
Figure 3 shows the disk-equivalent albedo for a single wavelength (282 nm) as a function of phase angle.
One of the unique properties of the LLAMAS dataset is
the coverage of phase angle from near zero to over 170
degrees. This will allow us to study both the opposition
surge well as surface roughness.
For comparison, we have included the phase curve
for visible wavelengths from the Robotic Lunar Observatory (ROLO) [6]. Both phase curves show asymmetry between the waxing and waning phases (positive
and negative angles, respectively). In the visible part
of the spectrum, mare appear darker than highlands terrain, thus the waning moon is on average darker than the
waxing moon due to a larger abundance of maria in the
western hemisphere. Preliminary indications are that the
asymmetry in the ultraviolet is in the opposite sense than
in the visible. This is consistent with observations by
Lucke et al. [7] that showed the relative brightness of
highlands and mare are reversed in the ultraviolet, i.e.
that mare is bright and highlands are dark.
Although we have a catalog of lunar observations
starting in 2006, we have only recently acquired resources to analyze this dataset. The results that we will
present at this meeting are therefore preliminary.
[1] W. E. McClintock, G. J. Rottman, and T. N. Woods. SolarStellar Irradiance Comparison Experiment II (Solstice II):
Instrument Concept and Design. Solar Phys., 230:225–
258, August 2005. doi: 10.1007/s11207-005-7432-x.
[2] G. Rottman. The SORCE Mission. Solar Phys., 230:7–25,
August 2005. doi: 10.1007/s11207-005-8112-6.
[3] B. Hapke. Bidirectional reflectance spectroscopy. IV - The
extinction coefficient and the opposition effect. Icarus,
67:264–280, August 1986. doi: 10.1016/0019-1035(86)
90108-9.
[4] B. Hapke. Theory of reflectance and emittance spectroscopy. 1993.
[5] M. Snow, G. Holsclaw, W. E. McClintock, and T. Woods.
Absolute ultraviolet irradiance of the moon from SORCE
SOLSTICE. In Proc. of the SPIE, volume 6677, October
2007. doi: 10.1117/12.732498.
[6] H. H. Kieffer and T. C. Stone. The Spectral Irradiance of
the Moon. AJ, 129:2887–2901, June 2005. doi: 10.1086/
430185.
[7] R. L. Lucke, R. C. Henry, and W. G. Fastie. Far-ultraviolet
albedo of the moon. AJ, 81:1162–1169, December 1976.
doi: 10.1086/112000.